Sewage sludge contains solid components, water, and additives. The sludge is fed through an infeed conveyor which transports the material into a sludge dryer housing that may comprise several drying zones. The drying gas is caused to envelope the sludge, whereby the drying sludge has a temperature that is kept below the activating temperature of the additives in the sludge. It is necessary to reduce the water or moisture content of the sludge to a moisture remainder that facilitates the further handling and/or treatment of the dried material.
German Patent Publication DE-OS 3,518,323 (Sevar), published on Nov. 27, 1986 discloses a method and apparatus for drying of sewage sludge, wherein the initially liquid sewage sludge is predried to form a sludge of a pasty consistency. The pasty sludge is then pelletized to form particles that have a relatively large surface, such as flat pellets or flakes. These large surface particles are then exposed to a drying gas flow. The known method wants to provide a dried sludge that has an easily adjustable remainder moisture content and that result is to be achieved with as small an energy input as possible. A remainder moisture content of around 5% by weight is supposed to be achieved in a drying apparatus that combines a predryer with a conveyor dryer passing through a housing in which the predried particles are exposed to a drying air stream. The drying air stream has a temperature of about 180.degree. C. when it enters the dryer housing and it exits the dryer housing at a temperature of about 80.degree. C. German Patent Publication 3,518,323 does not disclose, nor does it make any suggestion with regard to the recovery of the drying gas.
German Patent Publication DE-OS 4,013,761 (Sevar), published on Oct. 31, 1991, describes a method and apparatus quite similar to those described in the first mentioned German Patent Publication 3,518,323. As in the first mentioned publication, the drying gas stream also meanders repeatedly through the pelletized bulk material travelling on a screen conveyor. However, the temperature is to be kept so low that additives in the bulk material that frequently contain harmful substances will be retained in the bulk material, thus preventing the escape of harmful substances into the drying air. For this purpose the drying air is heated within a range of about 70.degree. to 75.degree. C. At these temperatures water condenses within the dryer housing. The condensate must be collected and treated in a settling pool, whereby harmful substances remain in the settling pool, where these substances cause a problem regarding their safe removal.